We studied at necropsy atherosclerotic plaque composition in the 4 major (right, left main, left anterior descending and left circumflex) epicardial coronary arteries in 15 patients who died of consequences of an acute myocardial infarction (AMI) and in 12 patients with sudden coronary death (SCD) without AMI. The coronary epicardial arteries were sectioned at 5-mm intervals, and a Movat stained section of each segment of artery was prepared and analyzed using a computerized morphometry system. Within the AMI group and within the SCD group, there were no differences in plaque composition among any of the 4 major epicardial coronary arteries. Within both groups plaque morphology varied as a function of cross-sectional-area narrowing of the segments. In both groups, the amount of dense relatively acellular fibrous tissue, calcified tissue, and pultaceous debris increased in a linear fashion with increasing degrees of cross-sectional-area narrowing of the segments and the amount of cellular fibrous tissue decreased linearly. In the AMI group, the percent of plaque consisting of pultaceous debris and of cellular fibrous tissue separated significantly narrowed (>75% cross sectional area) segments from less narrowed (<75%) segments. A comparison of the AMI group to the SCD group showed significant differences. The percent of plaque consisting of pultaceous debris (16% in the AMI group and 7% in the SCD group), of cellular fibrous tissue (11% -vs- 18%) and of heavily calcified tissue (8% -vs- 16%) were significantly different in the severely narrowed segments in the AMI and SCD groups. Occlusive coronary thrombi were present in 13 of the 15 AMI cases and in 1 of the 12 SCD cases. Thus, the frequency of coronary thrombi and plaque composition differ in patients with AMI and in those with SCD without AMI.